The invention relates to a boosted hydro-pneumatic drive for the rapid-traverse and power stroke modes, particularly for driving punching tools, consisting of a cylinder housing with partition, a disc piston with plunger, a working piston which can be loaded with compressed air for the return stroke, and an annular piston surrounding the plunger. The cylinder spaces between the working piston and the annular piston on both sides of the partition are filled with hydraulic oil.
Underlying state of the art is evidenced by U.S. Pat. No. 3,426,530 and U.S. Pat. No. 4,072,013.
In a drive of this class, known from U.S. Pat. No. 3,426,530, during rapid traverse compressed air is introduced into the space between the annular piston and the disc piston. The annular piston thereby displaces the supply of hydraulic oil and with the aid of the latter moves the working piston. The aforementioned space is simultaneously the space swept by the disc piston during the power stroke, and in rapid traverse forms a dead air space which is relatively large. This has the disadvantageous consequence that a great amount of compressed drive air is consumed during rapid traverse. In addition, the build-up and reduction in pressure requires a relatively long time in this large space, reducing the speed of response of this known drive. At the same time it must be considered that the drive air pressure of the rapid traverse must be removed completely for the power stroke since it opposes the starting movement of the disc piston.
Since the annular piston of the known drive is loaded on one side with compressed air and on the other side with hydraulic oil, it is provided with a special sealing arrangement consisting of two sealing rings and ventilation of the annular space between them. From experience, this is unavoidable since no seal is completely tight for long and since, therefore, air could get into the hydraulic oil and cause it to foam without ventilation of the interspace. The consequence of this would be that the required power could not be generated at the working piston and the drive could heat up to an inadmissible degree. Since, however, the ventilating opening in the cylinder housing does not move with the annular piston, the sealing rings must be spaced apart by at least the length of the stroke of this piston. This brings about a disadvantageous extension of the whole cylinder housing of the drive according to U.S. Pat. No. 3,426,530.
Finally, the construction of this known drive is of a complicated type in that the plunger is a hollow body into which projects a trunk-shaped projection of the front wall of the cylinder.
A boosted hydro-pneumatic drive of a different class is known from U.S. Pat. No. 4,072,013. In order to move the working piston in a rapid traverse, it is provided with an annular surface which can be loaded with compressed air in the direction of advance. Although this reduces the response time, a lot of compressed drive air is consumed by this known arrangement since an additional annular piston, surrounding the working piston and supported in its own cylinder housing, is loaded with compressed air. Furthermore, the concentric nesting structure of this known drive has essential disadvantages related to production engineering, which can also be seen from the great number of components required.
The present invention consists in a boosted hydro-pneumatic drive as follows
the working piston is provided with an annular surface which can be loaded with compressed air in the direction of advance,
a compression spring is clamped between the disc piston and the annular piston and
the space containing the compression spring is always connected to the atmosphere.
Such a drive consumes little compressed drive air per stroke, responds quickly, is of short constructional length and is easy to produce.
A considerable reduction in the consumption of compressed air is the result, on the one hand, of the elimination of all dead air space and, on the other hand, of the fact that the annular piston is no longer driven by means of compressed air but by means of the compression spring. Simultaneously there is an increase in the speed of response, that is to say a shortening of the time interval from the start signal to the end of the power stroke, an advantageous contributing factor to this being the compressing spring since the initial pressure effected by it in the oil reservoir supports the pneumatic drive of the working piston in a rapid traverse. The hitherto required dual sealing of the annular piston can be omitted since this is no longer loaded with compressed air. Rather, as mentioned, the hydraulic oil is under constant excess pressure, so that, at the most, oil can leak out but air can never get into the oil. Due to the elimination of the spaced dual sealing, the annular piston and thus the whole cylinder housing can be shortened considerably. Finally, the invention achieves a structural simplification by the elimination of both tubular and multiple concentrically nested elements, producing a noticeable advantage in production and thus in price.
Since the space containing the compression spring reduces its volume with every working stroke and increases it again, air is ejected and sucked in again from the environment. This breathing process produces an advantageous cooling of the drive. In order to keep the resistance to air flow low and to prevent dust and foreign bodies from entering into the space, it is proposed in further development of the invention that the connection to the atmosphere is made via a filter.